Catalyst electrode for fuel cell, manufacturing method thereof and a fuel cell comprising the catalyst electrode for fuel cell

ABSTRACT

Disclosed are a catalyst electrode for a fuel cell, a method for fabricating the catalyst electrode, and a fuel cell including the catalyst electrode. The presence of an ionomer-ionomer support composite in the catalyst electrode prevents the porous structure of the catalyst electrode from collapsing due to oxidation of a carbon support to avoid an increase in resistance to gas diffusion and can stably secure proton channels. The presence of carbon materials with high conductivity is effective in preventing the electrical conductivity of the electrode from deterioration resulting from the use of a metal oxide in the ionomer-ionomer support composite and is also effective in suppressing collapse of the porous structure of the electrode to prevent an increase in resistance to gas diffusion in the electrode. Based on these effects, the fuel cell exhibits excellent performance characteristics and prevents its performance from deteriorating during continuous operation.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a catalyst electrode for a fuel cell, amethod for fabricating the catalyst electrode, and a fuel cell includingthe catalyst electrode.

2. Description of the Related Art

Fuel cells are systems that convert chemical energy of fuel intoelectrical energy. Fuel cells use hydrogen gas and oxygen gas in the airas fuels. The hydrogen gas is produced by reforming or waterelectrolysis of a hydrocarbon fuel such as methanol or ethanol. Thehydrogen electrochemically reacts with the oxygen to form water. Thefuel cells based on this electrochemical reaction to produce electricityare considered as clean energy sources with high power and conversionefficiency compared to traditional internal combustion engines.

A typical fuel cell has a basic structure consisting of an anode, acathode, and a polymer electrolyte membrane. A catalyst layer on anodepromotes the oxidation of a hydrogen fuel and another catalyst layer oncathode promotes the reduction of an oxygen. The hydrogen fuel isoxidized at the anode to generate protons and electrons, the protons aretransferred to the cathode through the electrolyte membrane, and theelectrons are transferred to an external circuit through a wire. Theprotons transferred through the electrolyte membrane, the electronstransferred from the external circuit through the wire, and oxygencombine at the cathode to form water. The flow of the electrons passingvia the anode, the external circuit, and the cathode becomes electricpower. The catalyst contained in the cathode promotes theelectrochemical reduction of the oxygen and the catalyst contained inthe anode promotes the electrochemical oxidation of the fuel.

The performance of fuel cells is strongly dependent on the performanceof catalysts used in the anode and the cathode. Platinum (Pt) is themost widely used material for the catalyst electrodes. Particularly,Pt/C catalysts have recently been used as the most representativematerials for catalyst electrodes. Pt/C catalysts have a structure inwhich platinum particles are loaded on a carbon support with largespecific surface area and high electrical conductivity. It is importantto reduce the amount of the platinum loaded on the carbon supportbecause the platinum is an expensive precious metal. It is alsonecessary to maximize the performance of the catalysts by optimizing therelated factors such that effective loading of a small amount of theplatinum is achieved. In recent years, catalyst electrodes have beendeveloped in which particles of alloys of platinum (Pt) and othermetals, for example, transition metals such as nickel (Ni), palladium(Pd), rhodium (Rh), titanium (Ti), and zirconium (Zr), are loaded oncarbonaceous supports.

However, the carbon supports are prone to oxidation and lose theirperformance due to instability of the electrodes under electrochemicalconditions during operation of fuel cells. Thus, there is a need for asolution to the problem of poor long-term stability of carbon supportsencountered in the commercialization of fuel cell technology.

PRIOR ART DOCUMENTS Patent Documents

(Patent Document 1) Korean Patent Publication No. 10-2018-0079635

(Patent Document 2) Korean Patent Publication No. 10-2018-0040394

SUMMARY OF THE INVENTION

The present invention intends to prevent the performance of a fuel cellfrom deteriorating when a carbon support is degraded during operation ofthe fuel cell, causing collapse of the structure of a catalystelectrode, and an object of the present invention is to provide acatalyst electrode for a fuel cell including an ionomer-ionomer supportcomposite and carbon materials and a method for fabricating the catalystelectrode.

One aspect of the present invention is directed to a catalyst electrodefor a fuel cell including a carbon support loaded with metal catalystparticles, an ionomer-ionomer support composite, and carbon materialsselected from the group consisting of carbon nanotubes, carbonnanofibers, carbon nanorods, and mixtures thereof wherein theionomer-ionomer support composite includes an ionomer support includinga metal oxide and an ionomer covering the ionomer support.

A further aspect of the present invention is directed to a method forfabricating a catalyst electrode for a fuel cell, including (a)preparing a mixed solution containing a metal oxide and an ionomer, (b)stirring and drying the mixed solution to prepare an ionomer-ionomersupport composite, (c) adding the ionomer-ionomer support composite,carbon materials, and a carbon support loaded with metal catalystparticles to a solvent, followed by mixing to form an electrode slurry,and (d) fabricating a catalyst electrode using the electrode slurrywherein the carbon materials are selected from the group consisting ofcarbon nanotubes, carbon nanofibers, carbon nanorods, and mixturesthereof.

Another aspect of the present invention is directed to a fuel cellincluding the catalyst electrode.

The presence of the ionomer-ionomer support composite in the catalystelectrode of the present invention prevents the porous structure of thecatalyst electrode from collapsing due to oxidation of the carbonsupport to avoid an increase in resistance to gas diffusion and canstably secure proton channels. The presence of the additional carbonmaterials with high conductivity is effective in preventing theelectrical conductivity of the electrode from deterioration resultingfrom the use of the metal oxide in the ionomer-ionomer support compositeand is also effective in suppressing collapse of the porous structure ofthe electrode to prevent an increase in resistance to gas diffusion inthe electrode. Based on these effects, the fuel cell of the presentinvention exhibits excellent performance characteristics and preventsits performance from deteriorating during continuous operation.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects and advantages of the invention will becomeapparent and more readily appreciated from the following description ofthe embodiments, taken in conjunction with the accompanying drawings ofwhich:

FIG. 1A shows TEM images showing the dispersion state of a catalystslurry for a fuel cell formed in Example 4, FIG. 1B shows FIB-SEM imagesshowing the porous structure of an electrode fabricated from thecatalyst slurry using an ultrasonic sprayer, and FIG. 1C shows FE-SEMimages showing the thicknesses of electrodes for fuel cells fabricatedin Examples 1-4;

FIG. 2 compares the thicknesses of electrodes for fuel cells fabricatedin Examples 1-4 and Comparative Example 1;

FIG. 3A shows the I-V characteristics of catalyst electrodes for fuelcells fabricated in Examples 1-4 and Comparative Example 1 before ADT,FIG. 3B shows the I-V characteristics of the catalyst electrodes afterADT, FIG. 3C shows changes in the current densities measured at 0.6 Vbefore and after ADT in FIGS. 3A and 3B, and FIG. 3D shows changes inthe current densities measured at 0.4 V, where resistance to gasdiffusion predominantly affects, before and after ADT in FIGS. 3A and3B;

FIG. 4 shows the ECSA values of catalyst electrodes for fuel cellsfabricated in Examples 1-4 and Comparative Example 1 before and afterADT;

FIG. 5A shows Nyquist plots of catalyst electrodes for fuel cellsfabricated in Examples 1-4 and Comparative Example 1 before and afterADT and FIG. 5B shows the ohmic resistances and charge transferresistances of the catalyst electrodes before and after ADT;

FIG. 6A shows the initial oxygen gains (before ADT) of catalystelectrodes for fuel cells fabricated in Examples 1-4 and ComparativeExample 1, FIG. 6B shows the oxygen gains of the catalyst electrodesafter ADT, and FIG. 6C shows changes in the oxygen gains of the catalystelectrodes measured before and after ADT in FIGS. 6A and 6B; and

FIG. 7A shows surface FE-SEM images of a catalyst electrode for a fuelcell produced in Example 2 before and after ADT and FIG. 7B showssurface FE-SEM images of a catalyst electrode for a fuel cell fabricatedin Comparative Example 1 before and after ADT.

DETAILED DESCRIPTION OF THE INVENTION

Several aspects and various embodiments of the present invention willnow be described in more detail.

One aspect of the present invention provides a catalyst electrode for afuel cell including a carbon support loaded with metal catalystparticles, an ionomer-ionomer support composite, and carbon materialsselected from the group consisting of carbon nanotubes, carbonnanofibers, carbon nanorods, and mixtures thereof wherein theionomer-ionomer support composite includes an ionomer support includinga metal oxide and an ionomer covering the ionomer support.

A conventional fuel cell suffers from a rapid loss of performance withincreasing number of cycles because metal catalyst particles and/or acarbon support are lost by degradation under specific operatingconditions such as repetitive start up/shut down and a fuel starvation.Collapse of pores in a catalyst layer caused when the metal catalystparticles and/or the carbon support is degraded, surface hydrophilicitycaused by oxidation of the support, flooding in the catalyst layer areknown to be major causes of increased gas diffusion resistance of thefuel cell.

Particularly, since the performance of a fuel cell can be ensured onlywhen electron, proton, and oxygen channels are stably formed in anoxygen electrode (cathode), it is common that a porous electrodestructure is formed and an ionomer is contained therein to form protonchannels.

However, a loss of the carbon support caused by degradation leads tocollapse of the porous structure, which is the most fundamental problembecause oxygen channels cannot be secured and the ionomer cannot serveto improve the proton conductivity.

The present inventors have found that the ionomer-ionomer supportcomposite including the ionomer support can protect the porous structureof the electrode from collapsing due to carbon loss, unlike aconventional ionomer-containing catalyst electrode. The ionomer-ionomersupport composite imparts durability to the porous structure of thecatalyst electrode so that a substantial portion of the porous structurecan be maintained even when carbon support is lost by repeated drivingof the fuel cell. Simultaneously with this, the ionomer-ionomer supportcomposite serves as an appropriate dispersant for the ionomer toaccomplish two purposes: 1) good oxygen transmission and 2) improvedproton conductivity. Moreover, the presence of the carbon materialsselected from the group consisting of carbon nanotubes, carbonnanofibers, carbon nanorods, and mixtures thereof leads to improvementsin structural stability and electrical conductivity of the catalystelectrode.

The carbon materials are preferably carbon nanotubes. The carbonmaterials serve to improve the electrical conductivity of the electrodedue to their substantially high electrical conductivity. The carbonmaterials are arranged in random direction between the carbon supportloaded with metal catalyst particles and the ionomer-ionomer supportcomposite due to their small volume. This randomly directed arrangementis suitable to maintain the porous structure of the catalyst electrodeto reduce the resistance to oxygen diffusion.

The metal oxide may be selected from the group consisting of TiO₂, SnO₂,and CeO₂. The metal oxide is preferably TiO₂ for its lowest price, whichis advantageous in the commercialization of the catalyst electrode.

The ionomer may be Nafion.

The ionomer covering the ionomer support has a thickness of 0.5 to 10nm, preferably 1 to 2 nm. If the thickness of the ionomer is less than0.5 nm, the migration paths of protons in the electrode may be limited.Meanwhile, if the thickness of the ionomer exceeds 10 nm, the ionomerlayer tends to aggregate, the oxygen transmission rate of the ionomermay drop significantly, and the excessively large volume of the ionomermay make it impossible to form the desired porous structure of thecatalyst electrode.

The metal oxide has a diameter of 20 to 100 nm, preferably 20 to 30 nm.If the diameter of the metal oxide is less than 20 nm, theionomer-ionomer support composite tends to aggregate when introducedinto the catalyst electrode. Meanwhile, if the diameter of the metaloxide exceeds 100 nm, the decreased surface area may reduce theelectrochemical active area of the catalyst electrode, resulting in areduction in the catalyst utilization efficiency of the catalystelectrode. Particularly, the use of TiO₂ having a diameter in the rangeof 20 to 30 nm as the metal oxide is preferred in that the composite canbe prevented from aggregating, a sufficient electrochemical active areaof the catalyst electrode can be ensured, and the structure of thecatalyst electrode can be stably maintained despite repeated use.

The metal catalyst may be selected from the group consisting ofplatinum, ruthenium, osmium, platinum-palladium, platinum-rutheniumalloys, platinum-cobalt alloys, platinum-nickel alloys, platinum-iridiumalloys, platinum-osmium alloys, and mixtures thereof. Platinum ispreferred.

The carbon support may be selected from the group consisting of Vulcan,carbon black, graphite carbon, acetylene black, ketjen black, carbonfiber, and mixtures thereof.

The ionomer-ionomer support composite may be present in an amount of 28to 280 parts by weight, based on 100 parts by weight of the carbonsupport. If the content of the ionomer-ionomer support composite is lessthan 28 parts by weight, the pores of the catalyst electrode may not beeffectively maintained. Meanwhile, if the content of the ionomer-ionomersupport composite exceeds 280 parts by weight, the electricalconductivity of the electrode may be limited as a whole due to the lowelectrical conductivity of the ionomer support, the excessivelyincreased volume of the ionomer-ionomer support composite may lead to anincrease in the thickness of the electrode, and the pores of thecatalyst electrode may be blocked when assembled, resulting in increasedresistance to oxygen diffusion.

The ionomer-ionomer support composite is preferably present in an amountranging from 28 to 70 parts by weight. Within this range, a desired I-Vcurve and a sufficient electrochemical active area of the electrode canbe obtained and a significantly low ohmic resistance of the electrodecan be measured after accelerated degradation test (ADT). Particularly,the ohmic resistance decreases slowly with increasing amount of theionomer-ionomer support composite to 70 parts by weight, and thereafter,begins to increase rapidly with increasing amount of the ionomer-ionomersupport composite. For this reason, the content of the ionomer-ionomersupport composite is preferably limited to 28 to 70 parts by weight.

The carbon materials may be present in an amount ranging from 0.1 to5.0% by volume, based on 100% by volume of the carbon support. Withinthis range, the I-V characteristics of the catalyst electrode after ADTcan be improved. If the content of the carbon materials is less than0.1% by volume, an improvement in the electrical conductivity of thecatalyst electrode cannot be expected and the structure cannot beeffectively prevented from collapsing due to carbon corrosion.Meanwhile, if the content of the carbon materials exceeds 5.0% volume,the carbon materials may aggregate in the electrode, making it difficultto exert their normal performance, and proton channels of the ionomermay be blocked, with the result that the utilization efficiency of theplatinum catalyst is lowered, resulting in a reduction inelectrochemical active area.

More preferably, the carbon materials are present in an amount rangingfrom 0.5 to 1.5% by volume, based on 100% by volume of the carbonsupport. Within this range, the initial I-V characteristics of thecatalyst electrode and the I-V characteristics of the catalyst electrodeafter ADT can be improved compared to those of a commercial catalystelectrode including a Pt/C catalyst, the electrochemical active area ofthe catalyst electrode can reach a maximum, and the performancereduction and ohmic resistance of the catalyst electrode after ADT canbe effectively minimized Particularly, the initial I-V characteristicsof the catalyst electrode tend to be improved with increasing amount ofthe carbon materials within the above-defined range, and thereafter,begin to decrease rapidly outside the above-defined range. For thisreason, the content of the carbon materials is preferably limited to therange of 0.5 to 1.5 parts by volume.

Moreover, the initial oxygen gain of the catalyst electrode within theabove-defined range is low compared to that outside the above-definedrange. The initial oxygen gain is indicative of oxygen transmission.Particularly, the oxygen gain of the catalyst electrode is lower thanthat of a commercial catalyst electrode including a Pt/C catalyst,indicating improved gas diffusion in the electrode within theabove-defined range. The oxygen gain of a commercial catalyst electrodeincluding a Pt/C catalyst after ADT tends to increase significantlywhereas the oxygen gain of the electrode including the ionomer supportand the carbon materials after ADT tends to decrease.

Although explicitly described in the Examples section that follows, itwas found that it is important for the catalyst electrode of the presentinvention to meet the following conditions: (i) the metal oxide is TiO₂,(ii) the diameter of the TiO₂ is 20 to 30 nm, (iii) the ionomer-ionomersupport composite is present in an amount of 28 to 70 parts by weight,based on 100 parts by weight of the carbon support loaded with the metalcatalyst particles, and (iv) the carbon materials are present in anamount of 0.5 to 1.5% by volume, based on 100% by volume of the carbonsupport.

When the conditions (i) to (iv) were all met, the reduction rate of theoxygen gain of the catalyst electrode after ADT tends to increase withincreasing content of the carbon materials. If one or more of theconditions (i) to (iv) were not met, the reduction rate of the oxygengain of the catalyst electrode after ADT tends to decrease. In thisregard, it is important to meet all of the conditions in order tomaximize the desired effects of the present invention.

A further aspect of the present invention provides a method forfabricating a catalyst electrode for a fuel cell, including (a)preparing a mixed solution containing a metal oxide and an ionomer, (b)stirring and drying the mixed solution to prepare an ionomer-ionomersupport composite, (c) adding the ionomer-ionomer support composite,carbon materials, and a carbon support loaded with metal catalystparticles to a solvent, followed by mixing to form an electrode slurry,and (d) fabricating a catalyst electrode using the electrode slurrywherein the carbon materials are selected from the group consisting ofcarbon nanotubes, carbon nanofibers, carbon nanorods, and mixturesthereof.

The carbon materials are preferably carbon nanotubes.

The ionomer may be Nafion.

Step (a) may include (a-1) preparing a first mixed solution containing ametal oxide, (a-2) preparing a second mixed solution containing anionomer, and (a-3) mixing the first mixed solution with the second mixedsolution. The mixed solution prepared by mixing the first mixed solutioncontaining a metal oxide with the second mixed solution containing anionomer in step (a) is preferred in terms of dispersibility over a mixedsolution prepared by adding a mixture of a metal oxide and an ionomer toa solvent.

In step (a), the metal oxide may be mixed with the ionomer in suchamounts that the volume ratio is in the range of 1:0.3-2.0, preferably1:0.5-1.0. If the proportion of the ionomer is outside the above-definedrange, an excessively thick coating layer may be formed or the ionomeracting as a binder may aggregate.

The metal oxide may be selected from the group consisting of TiO₂, SnO₂,CeO₂, and mixtures thereof. The diameter of the metal oxide may be 20 to100 nm, preferably 20 to 30 nm. Effects related to the type and diameterof the metal oxide are the same as described above for the catalystelectrode and a detailed description thereof is thus omitted.

The mixed solution prepared in step (a) has a pH of 1 to 5, which ispreferable in terms of dispersibility. The pH of the mixed solution ispreferably 2 to 3 at which the highest dispersibility of the mixedsolution is attained.

The solvent of the mixed solution may be selected from isopropylalcohol, ethanol, and mixtures thereof.

The mixed solution may be dried at 50 to 200° C. for 10 to 24 hours.

Step (c) may include (c-1) sonicating a third mixed solution containingthe ionomer-ionomer support composite and the carbon materials for 1 to300 minutes and sonicating a fourth mixed solution containing a carbonsupport loaded with metal catalyst particles for 1 to 300 minutes and(c-2) mixing the third mixed solution with the fourth mixed solution andsonicating the mixture for 1 to 300 minutes to form an electrode slurry.The ionomer-ionomer support composite and the carbon materials can beuniformly dispersed without aggregation in the electrode slurry.

In step (c), the ionomer-ionomer support composite may be added in anamount of 2.5 to 6.5% by volume, based on the carbon support loaded withmetal catalyst particles. If the amount of the ionomer-ionomer supportcomposite is less than 2.5% by volume, the pores of the catalystelectrode may not be effectively maintained. Meanwhile, if the amount ofthe ionomer-ionomer support composite exceeds 6.5% by volume, theelectrical conductivity of the electrode may be limited as a whole dueto the low electrical conductivity of the ionomer support, theexcessively increased volume of the electrode may lead to an increase inthe thickness of the electrode, and the pores of the catalyst electrodemay be blocked when assembled, resulting in increased resistance tooxygen diffusion.

More preferably, the ionomer-ionomer support composite is added in anamount ranging from 5.0 to 6.5% by volume. Within this range, a desiredI-V curve and a sufficient electrochemical active area of the catalystelectrode can be obtained and a significantly low ohmic resistance ofthe catalyst electrode can be measured after accelerated degradationtest (ADT). Particularly, the ohmic resistance decreases slowly withincreasing amount of the ionomer-ionomer support composite to the upperlimit defined above, and thereafter, begins to increase rapidly withincreasing amount of the ionomer-ionomer support composite. For thisreason, the content of the ionomer-ionomer support composite ispreferably limited to 5.0 to 6.5 parts by weight.

In step (c), the carbon materials may be added in an amount ranging from0.1 to 5.0% by volume, based on the carbon support loaded with metalcatalyst particles. Within this range, the I-V characteristics of thecatalyst electrode after ADT can be improved. If the content of thecarbon materials is less than 0.1% by volume, an improvement in theelectrical conductivity of the catalyst electrode cannot be expected andthe structure cannot be effectively prevented from collapsing due tocarbon corrosion. Meanwhile, if the content of the carbon materialsexceeds 5.0% volume, the carbon materials may aggregate in theelectrode, making it difficult to exert their normal performance, andproton channels of the ionomer may be blocked, with the result that theutilization efficiency of the platinum catalyst is lowered, resulting ina reduction in electrochemical active area.

More preferably, in step (c), the carbon materials are added in anamount ranging from 0.5 to 1.5% by volume, based on 100% by volume ofthe carbon support loaded with metal catalyst particles. Within thisrange, the initial I-V characteristics of the catalyst electrode and theI-V characteristics of the catalyst electrode after ADT can be improvedcompared to those of a commercial catalyst electrode including a Pt/Ccatalyst, the electrochemical active area of the catalyst electrode canreach a maximum, and the performance reduction and ohmic resistance ofthe catalyst electrode after ADT can be effectively minimizedParticularly, the initial I-V characteristics of the catalyst electrodetend to be improved with increasing amount of the carbon materialswithin the above-defined range, and thereafter, begin to decreaserapidly outside the above-defined range. For this reason, the content ofthe carbon materials is preferably limited to the range of 0.5 to 1.5parts by volume.

Although explicitly described in the Examples section that follows,catalyst electrodes for fuel cells were fabricated by varying the orderof preparing the mixed solution containing the metal oxide and theionomer, the volume ratio between the metal oxide and the ionomer in themixed solution, the type and diameter of the metal oxide, the type ofthe ionomer, the pH of the mixed solution, the solvent of the mixedsolution, the drying temperature and time, the content of theionomer-ionomer support composite, and the type and content of thecarbon materials; and the surfaces of the catalyst electrodes wereobserved by high-magnification TEM.

As a result, when the following conditions (i) to (vii) were all met,the ionomer support was completely surrounded by the ionomer and theionomer layer was uniformly formed to the most preferred thickness of 1to 2 nm, as determined by high-magnification TEM, unlike when otherconditions and numerical ranges were employed: (i) step (a) includes(a-1) preparing a first mixed solution containing a metal oxide, (a-2)preparing a second mixed solution containing an ionomer, and (a-3)mixing the first mixed solution with the second mixed solution, (ii) themetal oxide and the ionomer are present in a volume ratio of 1:0.5-1.0in the mixed solution, (iii) the metal oxide is TiO₂, (iv) the diameterof the metal oxide is 20 to 30 nm, (v) the pH of the mixed solution is 2to 3, (vi) the solvent of the mixed solution is isopropyl alcohol, and(vii) the mixed solution is dried at a temperature of 50 to 200° C. for10 to 24 hours.

If one or more of the conditions (i) to (vii) were not met, the ionomersupport was not completely surrounded by the ionomer and was partiallyexposed or the ionomer layer was non-uniformly formed to a largethickness (≥2 nm) and its aggregation was observed.

Another aspect of the present invention provides a fuel cell includingthe catalyst electrode.

The following examples are provided to assist in further understandingof the present invention. However, these examples are provided forillustrative purposes only and the scope of the present invention is notlimited thereto. It will be evident to those skilled in the art thatvarious modifications and changes can be made without departing from thescope and spirit of the present invention.

Example 1. Production of Catalyst Electrode for Fuel Cell (PNTC0.5)

Preparation of Ionomer-Ionomer Support Composite

TiO₂ and an ionomer (Nafion) in a volume ratio of 1:0.5-1.0 wereseparately dispersed in isopropyl alcohol. Each of the mixed solutionswas dispersed by sonication for 10 min. The TiO₂-containing mixedsolution was adjusted to pH 2-3 by the addition of 1 M perchloric acid.The pH-adjusted TiO₂-containing mixed solution was mixed with theionomer-containing mixed solution, followed by additional sonication for30 min After sonication, the resulting mixed solution was dried in avacuum oven at 80° C. for 24 h to prepare an ionomer-ionomer supportcomposite.

Fabrication of Catalyst Electrode for Fuel Cell

A predetermined amount of the ionomer-ionomer support composite wasdispersed in isopropyl alcohol with a magnetic bar for 3 h, and furtherdispersed by sonication for 30 min (“mixed solution 1”). Highlyelectrically conductive carbon materials (carbon nanotubes) and a carbonsupport loaded with metal catalyst particles were dispersed in isopropylalcohol with a magnetic bar for 3 h and further dispersed by sonicationfor 30 min (“mixed solution 2”). The carbon materials were used in anamount of 0.5 vol % with respect to the volume of the carbon support.The mixed solution 1 was mixed with the mixed solution 2. The mixturewas dispersed by sonication for 30 min (“mixed solution 3”). Acommercial Pt/C catalyst (46.5 wt %, TKK) was dispersed in a mixedsolvent of distilled water and isopropyl alcohol and then apredetermined amount of an ionomer was added thereto (“mixed solution4”). The mixed solution 4 was dispersed by sonication for 30 min, mixedwith the mixed solution 3, and further dispersed by sonication for 30min. The resulting slurry was uniformly applied to one side of a polymerelectrolyte membrane using a sprayer until the platinum loading reacheda predetermined level (0.4 mg_(Pt) cm⁻²). This compartment correspondsto a cathode of a fuel cell membrane electrode assembly.

A commercial Pt/C catalyst (46.5 wt %, TKK) and an ionomer weredispersed in a mixed solvent of distilled water and isopropyl alcohol bysonication for 30 min. The dispersion was uniformly applied to theopposite side of the polymer electrolyte membrane using a sprayer untilthe platinum loading reached a predetermined level (0.2 mg_(Pt) cm⁻²).This compartment corresponds to an anode of the fuel cell membraneelectrode assembly.

Example 2. Production of Catalyst Electrode for Fuel Cell (PNTC1.25)

A catalyst electrode for a fuel cell was fabricated in the same manneras in Example 1, except that the carbon materials were used in an amountof 1.25 vol % with respect to the volume of the carbon support loadedwith metal catalyst particles.

Example 3. Production of Catalyst Electrode for Fuel Cell (PNTC2.5)

A catalyst electrode for a fuel cell was fabricated in the same manneras in Example 1, except that the carbon materials were used in an amountof 2.5 vol % with respect to the volume of the carbon support loadedwith metal catalyst particles.

Example 4. Production of Catalyst Electrode for Fuel Cell (PNTC5.0)

A catalyst electrode for a fuel cell was fabricated in the same manneras in Example 1, except that the carbon materials were used in an amountof 5.0 vol % with respect to the volume of the carbon support loadedwith metal catalyst particles.

Comparative Example 1. Production of Catalyst Electrode for Fuel CellUsing Commercial Catalyst (Pt/C)

A commercial Pt/C catalyst (46.5 wt %, TKK) and a predetermined amountof an ionomer were dispersed in a mixed solvent of distilled water andisopropyl alcohol by sonication for 30 min. The resulting catalystslurry was uniformly applied to one side of a polymer electrolytemembrane using a sprayer until the platinum loading reached apredetermined level (0.4 mg_(Pt) cm⁻²) This compartment corresponds to acathode of a fuel cell membrane electrode assembly. The catalyst slurrywas uniformly applied to the opposite side of the polymer electrolytemembrane until the platinum loading reached a predetermined level (0.2mg_(Pt) cm⁻²). This compartment corresponds to an anode of the fuel cellmembrane electrode assembly.

Experimental Example 1. HR-TEM and SEM Analyses

The catalyst electrodes for fuel cells fabricated in Examples 1-4 andComparative Example 1 were analyzed by TEM.

FIG. 1A shows TEM images showing the dispersion state of the cathodecatalyst slurry formed in Example 4, FIG. 1B shows FIB-SEM imagesshowing the porous structure of the electrode produced from the catalystslurry using an ultrasonic sprayer, and FIG. 1C shows FE-SEM imagesshowing the thicknesses of the electrodes fabricated in Examples 1-4.FIG. 2 shows the thicknesses of the electrodes produced in Examples 1-4and Comparative Example 1.

The images of FIGS. 1A, 1B, and 2 reveal that the ionomer-ionomersupport composite, the carbon materials, and the commercial Pt/Ccatalyst were uniformly dispersed in the slurries. Each of theelectrodes had a well-defined porous structure. The use of the carbonmaterials led to an increase in the thickness of the cathode. Thethickness of the cathode was increased by a maximum of ≥7 μm withincreasing volume of the carbon materials.

Experimental Example 2. Measurement of I-V Characteristics andDurability

A single cell including each of the catalyst electrodes prepared inExamples 1-4 and Comparative Example 1 was fabricated and evaluated forelectrochemical properties.

Specifically, the single cell was fabricated by assembling a Teflongasket and a gas diffusion layer (GDLs) on each of the anode and thecathode under a predetermined pressure. The gas diffusion layerconsisted of carbon paper and a microporous layer formed on the carbonpaper. The thickness of the gasket assembled on the cathode was changedto 240, 255, and 270 μm depending on the thickness of the cathode suchthat the same compressibility at the cathode was maintained. Thecompressibility at the cathode was found to be ˜1%.

The I-V characteristics of the single cell including the catalystelectrode fabricated in each of Examples 1-4 and Comparative Example 1were measured before and after ADT to find an optimal content of thecarbon materials. The measurement was done at 80° C., 1.8 bar, and 100%RH under H₂/Air atmosphere. The ADT was conducted at 80° C., 10 bar, and100% RH under H₂/Air atmosphere by applying a voltage of 1.3 V to thecell for 10 h.

FIG. 3A shows the I-V characteristics of the catalyst electrodesfabricated in Examples 1-4 and Comparative Example 1 before ADT, FIG. 3Bshows the I-V characteristics of the catalyst electrodes after ADT, FIG.3C shows changes in the current densities measured at 0.6 V before andafter ADT in FIGS. 3A and 3B, and FIG. 3D shows changes in the currentdensities measured at 0.4 V, where resistance to gas diffusionpredominantly affects, before and after ADT in FIGS. 3A and 3B.

As shown in FIGS. 3A to 3D, the performance characteristics of thecatalyst electrodes of Examples 1 and 2 before ADT were improvedcompared to those of the catalyst electrode of Comparative Example 1with increasing content (vol %) of the carbon materials, but theperformance characteristics of the catalyst electrodes of Examples 3 and4 before ADT were limited despite increasing amount (vol %) of thecarbon materials. The performance characteristics of the catalystelectrodes of Examples 1-4 before ADT were improved compared to those ofthe catalyst electrode of Comparative Example 1. The performancecharacteristics of the catalyst electrodes of Examples 1 and 2 after ADTwere improved compared to those before ADT, demonstrating that theperformance characteristics of the catalyst electrodes were limitedbefore ADT. Particularly, the catalyst electrode of Example 2 showed thehighest performance before ADT and underwent the least reduction inperformance after ADT, demonstrating that the content of the carbonmaterials in the catalyst electrode was most preferred. This differencewas more pronounced at 0.4 V, where resistance to gas diffusionpredominantly affects.

Experimental Example 3. Measurement of Electrochemical Active SurfaceArea (ECSA) Values

The electrochemical active surface area (ECSA) values of the single cellincluding the catalyst electrode fabricated in each of Examples 1-4 andComparative Example 1 were measured before and after ADT to find anoptimal content of the carbon materials.

FIG. 4 shows the ECSA values of catalyst electrodes fabricated inExamples 1-4 and Comparative Example 1 before and after ADT.

As shown in FIG. 4, the ECSA values of the catalyst electrodes ofExamples 1-4 showed a tendency to be substantially higher than that ofthe catalyst electrode of Comparative Example 1 due to the increasedelectrical conductivities by the use of the carbon materials, but theECSA values of the catalyst electrodes of Examples 3 and 4 showed atendency to gradually decrease with increasing content of the carbonmaterials. These results demonstrate that the excess carbon materialsaggregated in the electrodes to block channels in the ionomer, limitingthe migration of protons, and as a result, the utilization efficiency ofthe platinum catalyst is lowered, resulting in a reduction inelectrochemical active area.

Experimental Example 4. Measurement of Resistances

The resistances of the catalyst electrodes of Examples 1-4 andComparative Example 1 were measured.

FIG. 5A shows Nyquist plots of the catalyst electrodes fabricated inExamples 1-4 and Comparative Example 1 before and after ADT and FIG. 5Bshows the ohmic resistances and charge transfer resistances of thecatalyst electrodes before and after ADT.

As shown in FIGS. 5A and 5B, the ohmic resistances of the electrodes ofExamples 1-4 were lower than that of the electrode of ComparativeExample 1. This is believed to be because the use of the carbonmaterials substantially improved the conductivities of the electrodes.The ohmic resistance showed a tendency to increase in almost the samerate after ADT due to carbon corrosion. In contrast, the charge transferresistances of the catalyst electrodes of Examples 1 and 2 were lowerthan those of the catalyst electrode of Comparative Example 1 but showeda tendency to rapidly increase with increasing proportion of the carbonmaterials. This is believed to be because the excess carbon materialsblocked channels in the ionomer to limit the migration of protons, asdemonstrated in Experimental Example 3. The structure of the catalystelectrode of Comparative Example 1 collapsed and the ionomer aggregateddue to carbon corrosion after ADT, resulting in a significant increasein the charge transfer resistance of the catalyst electrode. Incontrast, the charge transfer resistances of the catalyst electrodes ofExamples 1-4 showed a tendency to decrease due to arrangement of theionomer during carbon corrosion.

Experimental Example 5. Measurement of Oxygen Gains

The oxygen gains of the single cells fabricated using the catalystelectrodes of Examples 1-4 and Comparative Example 1 were measuredbefore and after ADT to investigate the oxygen transmission rates of thecatalyst electrodes.

FIG. 6A shows the initial oxygen gains (before ADT) of the catalystelectrodes fabricated in Examples 1-4 and Comparative Example 1, FIG. 6Bshows the oxygen gains of the catalyst electrodes after ADT, and FIG. 6Cshows changes in the oxygen gains of the catalyst electrodes measuredbefore and after ADT in FIGS. 6A and 6B.

As shown in FIGS. 6A to 6C, the oxygen gains of the electrodes using thecarbon materials before ADT were found to be substantially lower thanthose of the electrode of Comparative Example 1, indicating improved gasdiffusion in the electrodes of Examples 1-4. The oxygen gains of thecatalyst electrodes of Examples 1 and 2 were much lower than those ofthe catalyst electrode of Comparative Example 1 but the oxygen gains ofthe catalyst electrodes of Examples 3 and 4 were comparable to orslightly higher than those of the catalyst electrode of ComparativeExample 1. This is believed to be because the excess carbon materialsmade the electrodes thick to cause deterioration of gas diffusion. Theoxygen gains of the catalyst electrode of Comparative Example 1 afterADT were much higher than those before ADT, whereas the oxygen gains ofthe catalyst electrodes of Examples 1-4 after ADT were lower than thosebefore ADT, which seems to be because the porous structures of thecatalyst electrodes were maintained during the carbon corrosion and theuse of the carbon materials to secure a larger number of gas channelsafter ADT than before ADT. Particularly, the oxygen gains of thecatalyst electrode of Example 2 both before and after ADT were lowerthan those of the other catalyst electrodes. In conclusion, the contentof the carbon materials in the catalyst electrode of Example 2 is mostpreferred.

Experimental Example 6. SEM Analysis

FIG. 7A shows surface FE-SEM images of the catalyst electrode fabricatedin Example 2 before and after ADT and FIG. 7B shows surface FE-SEMimages of the catalyst electrode fabricated in Comparative Example 1before and after ADT.

As shown in FIG. 7A, the surface morphology of the electrode of Example2, which was found to have the most preferred proportion of the carbonmaterials, after ADT was almost the same as that before ADT. Inconclusion, the porous structure of the catalyst electrode of Example 2suitable for oxygen transmission was maintained even after ADT.

In contrast, as shown in FIG. 7B, the porous structure of the catalystelectrode of Comparative Example 1 was hardly observed in the surfacemorphology of the catalyst electrode after ADT, unlike before ADT. Thisobservation reveals that the porous structure of the catalyst electrodeof Comparative Example 1 collapsed by oxidation of the carbon supportduring ADT.

As is apparent from the foregoing, the presence of the ionomer-ionomersupport composite in the catalyst electrode of the present inventionprevents the porous structure of the catalyst electrode from collapsingdue to oxidation of the carbon support to avoid an increase inresistance to gas diffusion and can stably secure proton channels. Thepresence of the additional carbon materials with high conductivity iseffective in preventing the electrical conductivity of the electrodefrom deterioration resulting from the use of the metal oxide in theionomer-ionomer support composite and is also effective in suppressingcollapse of the porous structure of the electrode to prevent an increasein resistance to gas diffusion in the electrode. Based on these effects,the fuel cell of the present invention exhibits excellent performancecharacteristics and prevents its performance degradation duringcontinuous operation.

Although the present invention has been described herein with referenceto the foregoing embodiments, these embodiments do not serve to limitthe scope of the present invention. Those skilled in the art willappreciate that various modifications are possible, without departingfrom the spirit of the present invention. Accordingly, the scope of thepresent invention should be defined by the appended claims.

What is claimed is:
 1. A method for fabricating a catalyst electrode for a fuel cell, comprising (a) preparing a mixed solution containing a metal oxide and an ionomer, (b) stirring and drying the mixed solution to prepare an ionomer-ionomer support composite, (c) adding the ionomer-ionomer support composite, carbon materials, and a carbon support loaded with metal catalyst particles to a solvent, followed by mixing to form an electrode slurry, and (d) fabricating a catalyst electrode using the electrode slurry wherein the carbon materials are selected from the group consisting of carbon nanotubes, carbon nanofibers, carbon nanorods, and mixtures thereof.
 2. The method according to claim 1, wherein step (a) comprises (a-1) preparing a first mixed solution containing a metal oxide, (a-2) preparing a second mixed solution containing an ionomer, and (a-3) mixing the first mixed solution with the second mixed solution.
 3. The method according to claim 1, wherein, in step (a), the metal oxide is mixed with the ionomer in such amounts that the volume ratio is in the range of 1:0.3-2.0.
 4. The method according to claim 1, wherein the metal oxide is selected from the group consisting of TiO₂, SnO₂, CeO₂, and mixtures thereof.
 5. The method according to claim 1, wherein the metal oxide has a diameter of 20 to 100 nm.
 6. The method according to claim 1, wherein the mixed solution prepared in step (a) has a pH of 1 to
 5. 7. The method according to claim 1, wherein, in step (c), the ionomer-ionomer support composite is added in an amount of 2.5 to 6.5% by volume, based on the carbon support loaded with metal catalyst particles.
 8. The method according to claim 1, wherein the carbon materials are added in an amount of 0.1 to 5.0% by volume, based on the carbon support loaded with metal catalyst particles.
 9. The method according to claim 1, wherein the carbon materials are present in an amount ranging from 0.5 to 1.5% by volume, based on the carbon support. 